Having special partial dispersion characteristics (: anomalous partial dispersion; Abbe number: 95) in addition to having extremely small chromatic dispersion, low refractive index and dispersion ratio compared to generic optical glasses, crystals of fluorite (CaF2 crystals) are used broadly in apochromatic lenses (apochromats), window plates of infrared analyzers, excimer lasers and the like, TV camera lenses and microscope lenses, lenses of semiconductor lithography (including steppers, scanners and the like) devices, which are devices for transferring microscopic patterns onto wafers, and the like.
Among them, regarding steppers (reduction projection-type exposure devices), which assume the miniaturization process in semiconductor lithography devices, shortening of the light source wavelength has been proceeding in order to raise the resolving power, and steppers have been developed, which use for the light source an excimer laser serving as a high output laser oscillating in the ultraviolet region, and concomitantly to this, fluorite (CaF2, calcium fluoride) has been drawing attention as a lens material suitable thereto. That is to say, a fluorite is characterized by the transmittance being high for light beams in a wavelength region called vacuum ultraviolet region, such as from a KrF laser (wavelength: 248 nm) or an ArF laser (wavelength: 193 nm), among the excimer laser beams. As one factor influencing the optical characteristics of a fluorite (calcium fluoride), refractive index homogeneity, or sub-boundary, which is a portion where dislocation has accumulated, can be cited.
As conventional technique focusing on such refractive index homogeneity, using a calcium fluoride (for instance Patent Document 1) having a refractive index homogeneity of 5 ppm (that is to say, 5,000 ppb) or lower and a birefringence of 10 nm/cm or lower, or a calcium fluoride (for instance Patent Document 2) having a refractive index homogeneity of 3 ppm (that is to say, 3,000 ppb) or lower and a birefringence of 2 nm/cm or lower in the exposure optical system has been proposed.
In addition, as a novel fluorite provided with a homogeneous CaF2 crystal with little strain birefringence (distortion), a fluorite is described in Patent Document 3, in which, when the light incidence plane is the (100) plane in a crystal substrate having parallel planes in the <100> direction, the mean value of the strain birefringence value per thickness at 633 nm wavelength is 0.4 nm/cm to 1.8 nm/cm, and the difference (PV) between the maximum value and the minimum value of the strain birefringence values per thickness at 633 nm wavelength is 4.0 nm/cm or less.
Meanwhile, lens materials for an excimer laser are sometimes damaged by so-called photodamages, in which a color center is formed within the material while being irradiated by a light beam such as from a laser, provoking a local change in refractive index due to a decrease in transmittance or absorption heating. Furthermore, when irradiated by a strong laser beam, not only the photodamages described above, but also destruction due to heat stress induced by absorption heating, or, damages due to insulation destruction, or the like, by the strong photoelectric field from the laser beam, are sometimes sustained, such that laser durability is one important evaluation item in this species of optical materials.
As conventional technique focused on such laser durability, for instance in Patent Document 4, a UV-compatible fluorite, in which the internal transmittance is 99.5%/cm or greater in a wavelength region of 150 nm or longer but 300 nm or shorter when irradiated with a pulsed laser beam in the ultraviolet light wavelength region for a number of pulses of 104 or more but 107 or less at an energy density of 1 mJ/cm2/pulse or more but 50 mJ/cm2/pulse or less, is described as a fluorite with excellent durability against ultraviolet light from KrF and ArF excimer lasers or the like.
In addition, in Patent Document 5, a calcium fluoride crystal, in which, after irradiating γ-ray at a dose of 1×105 R/hour for one hour, the amount of reduction of the internal transmittance per 10 mm thickness at 260 to 280 nm wavelength is 8% or lower before irradiation, is described as a calcium fluoride crystal with excellent durability against lasers such as excimer lasers.